delta13C fluctuations in precambrian to cambrian carbonate sequences in NE Brazil and NW Argentina

THEMES ON EARTH SCIENCES

Organizer:

A.C.Rocha-Campos

d¹³C FLUCTUATIONS IN PRECAMBRIAN TO CAMBRIAN CARBONATE SEQUENCES IN NE BRAZIL AND NW ARGENTINA

¹¹

ALEJANDRO J. TOSELLI ², FLORENCIO G. ACEÑOLAZA ²,

RICARDO ALONSO ³ and MIGUEL A. PARADA ⁴,

¹NEG-LABISE, UFPE, 50732-970 Recife, PE.

²INSUGEO, UNT, Argentina.

³Dept. de Geol., Univ. Nacional Salta, Argentina.

⁴Dept. de Geol., Univ. Chile, Santiago, Chile.

Carbon isotopes in carbonate sequences of Brazil and Argentina were analyzed in this study. In NE Brazil, rather unfossiliferous, weakly metamorphosed limestones of uncertain Neoproterozoic age are found in Ceará (Frecheirinha Fm) and Sergipe (Jacoca, Olho D'Agua and Acauã Fms) states. In Argentina, Upper Precambrian and Lower Cambrian, weakly metamorphosed sedimentary carbonates from two areas were studied here: (i) provinces of Salta (carbonates of the Lerma Group); (ii) province of San Juan (La Laja Fm., Precordillera; limestones intercalated with low-grade metapelites of the Pie de Palo complex, Precordillera basement).

The Frecheirinha Fm. shows negative C values in its lower portion (-2 ^o/_^ooPDB) and positive values up section (+1 to +3 ^o/_^ooPDB), a behavior observed in cap carbonates, deposited after glacial events 0.95Ga. The Jacoca and Acauã limestone Fms. show similar C (average - 5 ^o/_^ooPDB), compatible with deposition at 0.76Ga. In this region, the Olho D'Agua Fm. displays negative C values (-3 ^o/_^ooPDB) at its lower portion, shiftting up section to + 10 ^o/_^ooPDB, compatible with deposition 0.69Ga.

C in Las Tienditas carbonates (Lerma Group) shows the highest values (+3.4 ^o/_^ooPDB) at the basis of the section, decreasing gradually upsection. Clayey levels ocurr in the uppermost 15 m of the Formation. A negative excursion (-1.6 ^o/_^ooPDB) is observed in this interval and likely record the Precambrian-Cambrian transition (PCT). In the basis of La Laja Fm., C shows a negative (-2 ^o/_^ooPDB) excursion that is followed by a positive one (+0.5 ^o/_^oo). An anomaly (-2 ^o/_^ooPDB) is observed 240m from the basis of the Formation, and from 320m up, there is little variation (-0.5 to -0.1 ^o/_^ooPDB). The PCT seems to have been registered at the basis of this Formation.

Carbonates from three localities in the Pie de Palo complex display distinct C profiles: 0.0 ^o/_^ooPDB in the Pozo carbonates, from -1.0 to 0.0 ^o/_^ooPDB in Ciampo Venato, and from -1.4 to +1.3 ^o/_^ooPDB in San Ceferino. This narrow C-isotope variation is compatible with a Mesoproterozoic age deposition. (May 19, 2000)

Publication Dates

• Publication in this collection
  05 Jan 2001
• Date of issue
  Dec 2000